Geo Docs - Separate Easy from Advanced Install

What does this Merge Request do and why?

Since the addition of the One-Line installation of Geo it has become much easier for non-Geo team members to get Geo up and running without dealing with the configuration headache it can be to try and set up Geo manually on a non-fresh GDK.

However, there have been a few instances over the past few months where during code reviews, team members have been losing days trying to get Geo up and running. After a few calls/chats with different team members it seems most of the failure and struggles involve the step If you have one working GDK already: How to add a secondary Geo site.

While on the surface this setup may seem easier than the one-line it appears to be causing more issues than good.

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For this change I am proposing moving the "If you have one working GDK" and "Manual installation" guidelines to a new "Advanced Installation" document. Leaving just the easy (one-line) installation as the user path to get Geo running on the GDK. Working from this script should help team members avoid the configuration headaches that seem to come up again and again.

For the empowered user the advanced documentation still exists and should be able to be followed for more specific installation cases.

cc/ @geo-team

How to test

1. Review changes on docs: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/0714f141fe146990f388ab10e39562715ae8e4d8/doc/howto/geo.md

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• [-] Add the ~highlight label if this MR should be included in the CHANGELOG.md.